Breast cancer is a cause of significant morbidity and mortality in women, and is one of the most common causes of death due to cancer in women. Although prognosis for early stage breast cancer has improved over the years, treatment options for advanced stage metastatic breast cancer patients remain limited, with the five- year relative rate of survival of these patients being only 23%. One reason for this treatment deficit i that the underlying biology of metastasis remains poorly understood. It has been found that expression of huntingtin- interacting protein1 family members (Hip1 and Hip1r), are increased in several metastatic cancers, including prostate, colon, brain and breast cancer. Furthermore, Hip1 deficiency was shown to inhibit prostate tumor progression in mice, and having increased levels of Hip1 was demonstrated to be prognostic in prostate cancer. The proposed research focuses on the Hip proteins and addresses the hypothesis that the Hip proteins are directly involved in breast cancer metastasis in cancer cells that over-express Hip. The Hip proteins link membrane traffic to the actin cytoskeleton and preliminary data in the Brodsky lab has implicated this interaction in cell migration. The Hip proteins bind both the light chain subunits f clathrin, and actin, and the interaction between Hip and the clathrin light chains (CLCs) is suspected to play a key role in actin reorganization and cell motility in migrating tumor cells. Moreover, previous studies in the Brodsky lab have demonstrated that this interaction is targetable with a small peptide, and thus inhibition of this interaction to prevent tumor cell metastasis may provide a basis for a novel therapeutic drug design. This proposal investigates the role of the Hip protein-CLC interaction in cancer cell migration in advanced stage breast cancer. Later aims focus on defining characteristics of small peptide inhibitors of the Hip-CLC interaction. The first aim will examine the role of the Hip protein and CLC protein-protein interaction during cell migration in human breast cancer cells. The second aim will determine the consequences of genetically inhibiting this protein-protein interaction in breast cancer cell metastasis in immuno-deficient mice. Lastly, the third aim will define the effects of inhibiting th Hip protein-CLC interaction on migration using small peptide inhibitors in human breast cancer cells. Taken together, these studies will define a novel molecular mechanism involved in breast cancer metastasis and identify the basis of new a therapeutic approach to control metastatic breast cancer.